The present invention relates to water reactor internal structures having a titanium material coating over a zirconium base alloy. It is especially concerned with the cladding of water reactor fuel rods.
Nuclear fuel cladding for light water reactors requires resistance to corrosion in high temperature, high pressure aqueous liquid and/or steam conditions. In addition, nuclear fuel cladding also requires adequate structural mechanical properties and a low thermal neutron absorption cross section.
The initial material chosen for light water reactor fuel cladding was zirconium. However, it was soon found that a material possessing improved strength and corrosion resistance compared to zirconium would be desirable. Development work to improve the corrosion resistance and/or strength of zirconium fuel cladding may be divided into two general areas: (1) those efforts seeking to improve corrosion resistance and/or strength by making alloying element additions; and (2) those efforts seeking to improve corrosion resistance and/or strength by coating the zirconium cladding with a non-zirconium base material. The efforts in the area of zirconium base alloys eventually were fruitful and have resulted in a number of commercial zirconium base alloys for use as nuclear fuel cladding. These commercial alloys include Zircaloy-2 and Zircaloy-4.
The development efforts in the area of coated zirconium fuel cladding were, to our knowledge, unsuccessful. These efforts, however, included looking at coatings of beryllium, Inconel, Monel (Inconel and Monel are, respectively, trademarks of INCO Alloys International, Inc. for their nickel-chromium and nickel-copper alloys), nickel, iron, molybdenum, platinum, silver, 18-8 stainless steel, tantalum and titanium. Of all the foregoing materials only titanium could produce a ductile metallurgical bond with zirconium that did not contain brittle intermetallic compounds. However, the thermal neutron absorption cross section of titanium is significantly higher than that of zirconium (approximately 5.6 (barns) for titanium and 0.18 for zirconium). In addition, the investigators reported that the aqueous corrosion properties of titanium were not much better than those of zirconium.
Zircaloy-2 and 4 because of their low thermal neutron cross section (about 0.22 barns), high strength and relatively good aqueous corrosion resistance, have remained the alloys of choice for commercial light water reactor fuel cladding and structural members. The corrosion resistance of these Zircaloy components has been further improved by heat treatments, such as beta quenching.
The present inventors have now found, contrary to the aforementioned results of the prior investigators, that titanium and its alloys possess significant advantages over Zircaloy in the area of aqueous corrosion resistance. Therefore, in accordance with the present invention, an elongated nuclear fuel cladding tube, or other nuclear component, is provided which has an inner layer of a zirconium base alloy, over which is a second layer of titanium material selected from the group consisting of titanium, titanium alloys, titanium compounds, and their combinations with each other.
The composition of the titanium material has been selected such that the aqueous long term corrosion rate of the titanium material in 680.degree. F., 2700 psi water is about 1/10 or less of the aqueous post transition corrosion rate of Zircaloy-4 under the same conditions. More preferably, the corrosion rate of the selected titanium material is about 1/100, or less, that of Zircaloy-4 under the foregoing conditions.
Preferably the layer of titanium material has a wall thickness of less than about 0.004 inches, more preferably less than about 0.001 inches, and most preferably less than about 0.0005 inches. Preferably the layer of titanium material is at least about 0.000014 inches thick.
The foregoing and other aspects of the present invention will become more apparent upon review of the drawings, which are briefly described below, in conjunction with the following detailed description of the invention.